Arterial Involvement in Inferoposterior MI with 3rd Degree AV Block
The right coronary artery (RCA) is typically involved in inferoposterior myocardial infarction with 3rd degree AV block, as it supplies both the inferior wall of the left ventricle and the AV node in most patients.
Anatomical Basis
- The RCA is the culprit vessel in the majority of inferoposterior MIs complicated by complete heart block, particularly when the occlusion is proximal 1, 2
- In patients with a dominant RCA (approximately 85% of the population), this artery supplies the AV node through the AV nodal artery, making AV block a common complication of proximal RCA occlusion 1, 3
- The sinoatrial nodal artery, which provides major blood supply to the atria, also typically arises from the proximal RCA, explaining the association with atrial involvement 1
Clinical Characteristics of RCA Occlusion with AV Block
When 3rd degree AV block complicates inferior MI from RCA occlusion, it typically occurs at the AV node level and presents with distinct features:
- The block usually develops early (within 6 hours of symptom onset) and is often transient, resolving within 24 hours 4
- The escape rhythm is typically narrow-complex with rates of 40-60 bpm, indicating a junctional escape originating at or near the AV node 5
- Atropine may be effective in this setting because the block occurs at the AV node level where vagal tone plays a significant role 1, 4
Right Ventricular Involvement
- Right ventricular infarction accompanies inferoposterior MI in 22-53% of cases when the RCA is occluded proximally 2, 3
- Patients with RV involvement and 3rd degree AV block have significantly higher rates of hemodynamic compromise, including hypotension (48% vs 23%), cardiogenic shock, and need for temporary pacing 2
- High-degree AV block (Mobitz II and complete heart block) occurs significantly more frequently with RV involvement (22% vs 5%) 2
Important Clinical Distinction
The location of infarction determines both the mechanism and prognosis of AV block:
- Inferior MI (RCA territory): AV block occurs at the nodal level, may be transient, and can have favorable long-term outcomes if the block resolves, though in-hospital mortality remains elevated 1
- Anterior MI (LAD territory): AV block occurs infra-Hisian with extensive myocardial necrosis, carries an ominous prognosis, and reflects extensive damage rather than isolated electrical dysfunction 6
Management Implications
- Although AV block in inferior MI may resolve spontaneously, permanent pacing may be considered if symptomatic high-degree or 3rd degree heart block persists and does not resolve 1
- The decision for permanent pacing should not be made immediately, as the block is often transient in the setting of inferior MI from RCA occlusion 1
- Temporary pacing is indicated for symptomatic bradycardia or hemodynamic compromise while awaiting resolution 1, 7
Common Pitfall
Do not assume that all 3rd degree AV blocks in MI require permanent pacing—the anatomic location of both the infarct and the block level are critical determinants of management and prognosis 1.